This application is related to application Ser. No. 178,721, pending filed concurrently herewith.
Gas turbine engines typically include a core engine with a compressor for compressing air entering the core engine, a combustor where fuel is mixed with the compressed air and then burned to create a high energy gas stream, and a first turbine which extracts energy from the gas stream to drive the compressor. In aircraft turbofan engines a second turbine or low pressure turbine located downstream from the core engine extracts more energy from the gas stream for driving a fan. The fan provides the main propulsive thrust generated by the engine.
The rotating engine components of the turbine and compressor include a number of blades attached to a disc which are surrounded by a stationary shroud. In order to maintain engine efficiency, it is desirable to keep the space or gap between the tips of the blades and the shroud to a minimum. If the engine were to operate only under steady state conditions, establishing and maintaining a small gap would be fairly straight forward. However, normal operation of aircraft gas turbine engines involves numerous transient conditions which may involve changes in rotor speed and temperature. For example, during takeoff rotor speed and temperature are high which means that there is a correspondingly high radial expansion of the blades and disc. Similarly, during decreases in engine rotor speed and temperature there is a reduction in the radial size of the blades and disc. The stationary shroud also expands or contracts in response to changes in temperature.
It is difficult to devise a passive system in which the blades and disc move radially at the same rate as the shroud so as to maintain a uniform gap therebetween. This is due in part to the fact that the rotor grows elastically almost instantaneously in response to changes in rotor speed with essentially no corresponding shroud growth. Furthermore, there is a difference in the rate of thermally induced growth between the shroud and rotor. Typically, the thermal growth of the rotor blades lags the elastic growth, and thermal growth of the shroud lags blade thermal growth with disc thermal growth having the slowest response of all.
In the past, various active systems have been employed to control the relative growth between the shroud and rotor and thereby control the gap, for example, selectively heating and/or cooling the stator shroud such as disclosed in U.S. Pat. No. 4,230,436, Davison.
A proposal for controlling clearances in a compressor by selectively heating its rotor is disclosed in U.S. Pat. No. 4,576,547, Weiner. The system disclosed therein shows two sources of relatively high pressure compressor air at different temperatures being selectively admitted into the rotor bore at a mid stage station of the compressor. Control of clearances by continuously cooling a rotor is disclosed in U.S. Pat. No. 3,647,313, Koff.
As a further consideration, not only must an active system have the inherent capability to vary the clearance between the blade tip and surrounding shroud, but the control logic must accurately predict the clearance and send a signal to the means employed to vary the clearance.
An example of control logic used in a prior clearance control system is disclosed in U.S. Pat. No. 4,230,436--Davison. Davison controls two sources of air as a function of timing and engine speed. Other systems have also utilized engine speed as a control parameter. For example, U.S. Pat. No. 4,069,662--Redinger turns shroud cooling air on at a predetermined engine speed and altitude.
In a system where air temperature or flow can be more fully modulated more complex control logic may be required.